The invention is concerned with a device for measuring certain properties of particles suspended in a particle suspension with a first chamber, to which particle-free electrolyte is supplied, a separating wall and a second chamber, which is connected with the first chamber by means of a measuring aperture in the separating wall, with a pressure in the second chamber, which for generating the flow of a stream of particle-free electrolyte through the measuring aperture is less than the pressure in the first chamber, and a particle supply capillary in the first chamber in front of the measuring aperture, out of which the particle containing particle suspension is introduced into said stream, to transport the particles through the measuring aperture such that upon passage of a particle through the measuring aperture a measuring impulse is obtained between a first and a second electrode, which are provided in the first and in the second chamber respectively (U.S. Pat. No. 2,656,508).
With these devices the problem exists that some of the particles, which have passed the measuring aperture and have entered the second chamber, separate from the main flow of the stream of electrolyte. Such separation can be effected for example by the formation of vortices or turbulence. These particles then can return to the downstream side of the measuring aperture. If they reenter the area around the downstream end of the measuring aperture, in which already a concentration of the electrical field lines is present, they can induce impulses between the electrodes. These impulses are smaller than those, which the same particles would effect when passing through the measuring aperture. However, they are large enough to be included in an evaluation of the impulses, e.g. by classification in accordance to their height, since e.g. an impulse induced by a backwards travelling erythrocyte can have approximately the same height as the one induced by a thrombocyte upon passage of the measuring aperture. This means that the evaluation of the impulses is erroneous, when impulses as generated by backward travelling particles are evaluated as actually measuring impulses as generated by normal passage of particles through the measuring aperture.
For solving this problem of the backward travelling particles it has been suggested (U.S. Pat. No. 3,299,354, col. 2, lines 24-30) to provide downstream behind the measuring aperture a suction capillary, the entrance of which serves as a secondary aperture, through which the particle stream entering the second chamber is sucked in order to completely separate the particles from the second chamber shortly after they have entered it and thereby avoid the occurrence of particles travelling back to the measuring aperture. Within this suction capillary by suitable geometrical construction it is provided that these backward travelling particles are collected at a certain point such that they cannot again travel back through the secondary aperture (see op. cit., reference a numeral 44, col. 5, line 20). It is a disadvantage of this arrangement that special provisions are necessary to clean the suction capillary, in which the secondary aperture is provided (op. cit., col. 5, line 69 ) to col. 6, line 8). Also, this device is of an intricate and complicated construction.
Further devices are known, with which for each particle two measuring impulses are generated in a certain timely sequence upon passage through two apertures (DT-OS No. 21 11 356). With these devices, in the space between both measuring apertures, particle-free electrolyte is supplied. However, once a particle has passed the second measuring aperture, the backward travelling particles problem still exists in the same manner. No solution of this problem has been suggested.
It is the objective of the present invention to improve the device as aforementioned, in which the backward travelling particles problem, as explained, shall be avoided. In particular, the particles, which have entered the second chamber, shall be prevented from reaching the neighbourhood of the downstream end of the measuring aperture. This objective shall be accomplished without the necessity of providing behind the aperture a suction capillary with a secondary aperture, since this is considered to be constructively too complicated and difficult to clean.
In accordance the invention a device as aforementioned has the following improvements:
That in the direction of stream flow downstream behind the end of the measuring aperture means are provided for additional supplying particle-free electrolyte to combine with said flowstream to surround the same and deflect particles, which, in the second chamber, travel backward to the downstream end of the measuring aperture.
By providing additional outlets of particle-free electrolyte, preferably from radially extending channels, at a position downstream after the mesuring aperture, an additional flow of electrolyte is provided, which surrounds the particle stream at this position, whereby this additional electrolyte flow keeps away the backward travelling particles from the measuring aperture. This additional flow or stream serves as a sort of screening of the downstream end of the measuring aperture. This solution is extremely simple and obviates the necessity of providing special suction means to transport and separate the particle stream. It further has the advantage that the same means can be used to clean the measuring aperture by rinsing or flushing in those cases, in which particles stick to the same. Such flushing effect, to some extend, is achieved simultaneously with screening the measuring aperture. Further improvement will become apparent from the description.